My theory: At the time of the original releases in the late 70s/early 80s the concept was that either all Jedis, or else all powerful Force users that come to realize the peace promoted by the Jedi teachings, would have their mental will preserved within the Force and would become Force ghosts, period. By realizing this peace, you become one with the Force, there was no special knowledge, teaching, or special disappearing-upon-death required, and thus with your will preserved, could apparition to anyone sensitive to the Force.
I think the Force users that disappeared already had acquired this zen and were aware of their imminent deaths (Obi-Wan allowed Vader to cut him down, Yoda was aware due to the heightened sense provided by the Force) and manipulated the Force into allowing their bodies to disintegrate into the Force to either avoid the pain of death, or believed it a more "holy" way to pass (for lack of a better word). Anakin's love for his son made him realize this peace, allowing himself to be turned into a Force ghost after he died, but hadn't known it long enough to allow his body to dissipate. Again, let me state, I believed this was the case for the initial releases only.
Because then Episodes I - III were released, followed by another set of re-releases of IV - VI with additional tweaks and changes from the 1997 re-releases. The concepts and ideas presented in I - III irked many fans, and the additional changes in IV - VI further stabbed fans in the heart because, like most people, they can't handle change and felt their childhood's were being raped. It was in these new episodes and new round of changes that introduced and provided more complex information and understandings of how the Force worked, and people didn't like it.
There's a concept that Lucas talked about that gave me a little more peace when it came to the changes and tweaks that have constantly been made with every re-release of the series (Like Anakin's younger form as the Force ghost being a one of many for instances), which is Lucas believes his true vision of how the films being depicted couldn't fully be completed in the beginning, and every change is a reflection of the complete story he initially wanted to depict. That, like many books and plays, the initial versions were rough drafts or "rough cuts" of the ultimate final version of these films, and each change is closer to the actual, true realization of his story of the tragedy of Anakin Skywalker. That makes a lot of sense to me. While the classic releases will always be the nostalgic pieces of my childhood that they are, I accept that the minor and major tweaks and changes aren't perversions of what I held as true, but more like evolved concepts of the unexplained or vague ideas that I had always wondered about, or had a personal idea of that I can admit I was wrong about considering the explanations George Lucas provides.
(And since someone mentioned Jar Jar as a blasphemy, I would like to say that he may have been an annoying character probably created solely for children, but him being an idiot and becoming a senator greatly reflects on our modern day politicians, ESPECIALLY when you considered he's directly responsible for the legislation that all but ended the Jedi Order. It makes him the second greatest tragic character in the series, if you ask me, and more than justifies his existence).
I know I kind of got off on a tangent there but my point is that the changes between why Jedis ghost, how they died, who disappeared and who didn't, have definitely changed based on how we remember them from the initial releases and that isn't a bad thing.